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Full text of "The Flow Of Gases In Furnaces"

52

APPLICATION OF THE LAWS OF   HYDRAULICS

The type of roof which should be chosen—ascending, descend-
ing or horizontal—will depend upon the result which is desired
from the furnace.
1.  A horizontal roof will give a high temperature at the firebox
end of the hearth and a low temperature at the rear end of the
furnace.
2.  If a higher temperature is desired near the firebox end of the
furnace,  and a diminishing temperature toward the rear end,
the descending roof should be used.    The firebox end will form a
high chamber, in which the flaming gases will remain for a short
period of time and where they will complete their combustion.
The hot products of combustion will drop below the hotter flame,
and as they gradually give off their heat and become cooler, will
descend in horizontal layers.    The coolest gases will pass from the
heating chamber by means of the waste gas port in the hearth of
the furnace.
3.  In certain cases it is necessary to maintain a uniform tempera-
ture throughout the length of the furnace; it is then necessary to
use an ascending roof.
In the author's personal practice the following case came up
for analysis: two steel boxes for the annealing of steel sheets were
heated upon the hearth of a furnace. The first box heated much
more rapidly than the second box, and the annealing of the sheets
was accomplished at a temperature which was much too high.
This peculiarity of the furnace caused considerable trouble to the
plant, and it was impossible to hold back the annealing of the
first box so that the annealing of the second box would be com-
pleted at the same time. The furnace was changed so that the
annealing of both boxes was completed at the same time and tem-
perature, by replacing the descending roof of the furnace by a
horizontal roof. This was done because the author had a ground-
less fear, at this time, of making an ascending roof, which would
have carried the total mass of the hot gases to the rear end where
they would have distributed themselves very effectively and
uniformly throughout the full length of the furnace.
Vm. VELOCITY IN INVERTED WEIRS.   RATIONAL CONSTRUCTION
OF FURNACES OF THIS TYPE
Yesrnann's formula provides a method of computing the
normal velocity of the current of hot gases in an inverted